Denaturation of circular dsDNA - stupid, but how does it work? (Aug/10/2006 )

Hello all,

just preparing an important exam. During my reading I encountered
a 'problem' I never even thought of...

Some time ago I PCR-amplified whole plasmids for mutagenesis.
So, during the denaturing step at ~95 °C for PCR: how can you separate
the strands of circular dsDNA - I mean, they're wrapped around each
other, no nicks... just trying to imagine how that should work though
obviously it does?
Furthermore I always thought it's the principle of the alkaline lysis method
that plasmids renature faster than chromosomal DNA because their
DNA strands are held in close proximity when denatured...

Hope you understand what I mean. Thanks for suggestions!

i dont know if i got the question correctly, but i never thought of that before.
when you do a PCR the individual strands come apart from their complementary partner, i.e the double helix unwinds. if there are no nicks i assumed that its like two interlocked rings which i guess should still allow for replication. the pol should still be able to work on that template.
perhaps someone with more molecular biology can give an in depth answer.

This is one of those questions that keeps coming up. An instructor of mine, she said it was because the primers were so small that they were much faster to anneal than the original complementary strand at that temp (they sort of "sneak in")--the annealing step in a PCR is still mighty warm, so she might be right, unless someone has a more concrete answer . . .

Thank you very much for your answers, soraya and Meres!
That helps.